Dual Carbamoylations on the Polyketide and Glycosyl Moiety by Asm21 Result in Extended Ansamitocin Biosynthesis

SummaryCarbamoylation is one of the post-PKS modifications in ansamitocin biosynthesis. A novel ansamitocinoside with carbamoyl substitution at the C-4 hydroxyl group of the N-β-D-glucosyl moiety was identified from the ansamitocin producer, Actinosynnema pretiosum. Through biotransformation, the carbamoyltransferase gene asm21 was suggested to be responsible for the carbamoylation of the glucosyl moiety. Three new derivatives without the backbone carbamoyl group were isolated from an asm21 mutant and characterized by NMR spectroscopy. Among them, 18-O-methyl-19-chloroproansamitocin was the major product and the preferred substrate for macrolactam C-7 carbamoylation by Asm21. However, Asm21 exhibited higher catalytic efficiency toward the glucosyl moiety. Furthermore, the dual carbamoylations and N-glycosylation were precisely demonstrated in vivo. This work represents the first biochemical characterization of an O-carbamoyltransferase performing dual actions on both a polyketide backbone and a glycosyl moiety during ansamitocin biosynthesis

Testing the light scalar meson as a non-qqˉq\bar q state in semileptonic DD decays

To distinguish between the normal $q\bar q$ and exotic diquark-antidiqark ($q^2\bar q^2$) contents of the lowest-lying scalar meson ($S_0$), we investigate the semileptonic $D\to S_0 e^+\nu_e, S_0\to M_1 M_2$ decays, where $M_{1(2)}$ represents a pseudoscalar meson. With the form factors extracted from the current data, we calculate ${\cal B}(D_s^+\to \sigma_0 e^+\nu_e,\sigma_0\to\pi^0\pi^0) =(12.9^{+6.3}_{-4.9})\times 10^{-4}$ and $(0.8^{+1.2}_{-0.7})\times 10^{-4}$ for the $q\bar q$ and $q^2\bar q^2$ quark structures, respectively, and compare them to the experimental upper limit: $6.4\times 10^{-4}$. It is clearly seen that $S_0$ prefers to be the $q^2\bar q^2$ bound state. Particularly, ${\cal B}_{q\bar q}(D_s^+\to \sigma_0 e^+\nu_e,\sigma_0\to\pi^+\pi^-) =(25.8^{+12.5}_{-\;\,9.8})\times 10^{-4}$ and ${\cal B}_{q^2\bar q^2}(D_s^+\to \sigma_0 e^+\nu_e,\sigma_0\to\pi^+\pi^-) =(1.5^{+2.4}_{-1.3})\times 10^{-4}$ are predicted to deviate far from each other, useful for a clear experimental investigation.Comment: 10 pages, 1 figure, 1 tabl